1. Field of the Invention
The invention relates to a novel method for producing phosphorus-containing compounds which are effective as flame retardants.
For a long time, phosphorus compounds have been known to be flame retardants. In recent years, they have frequently also included 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-one or -oxide (DOPO), which was described for the first time by Sanko Chemical Co Ltd in DE 20 34 887, and various derivatives thereof. Their flame retardant properties appear to be due to the release by them of phosphorus-containing radicals when heated (see, for example, Schäfer et al., J. Appl. Polym. Sci. 105(2), 685-696 (2007)).

Derivatives of diphenylphosphine oxide (DPhPO) and diphenylphosphite (DPhOPO) are also known to be phosphorus compounds with a flame retardant effect; the mode of action is similar.
On the other hand, melamine and guanamine derivatives are among known nitrogen-containing compounds with a flame retardant effect:

whereupon experiments were carried out to combine such phosphorus-containing and nitrogen-containing compounds into flame retardants. Attempts were also made to produce covalent compounds between such molecular groups.
In US 2003/120021 A1, which corresponds to U.S. Pat. No. 6,797,821 B2, and US 2005/0004339 A1 from Wang et al., curing agents for epoxy resins or epoxy resins cured therewith are described which each contain one or more DOPO or diarylphosphine oxide groups covalently bonded to nitrogen-containing molecules, including melamine, methyl and phenyl guanamine and have the following formula:

in which Q′ could be, inter alia, a DOPO or DPhPO residue, R could be NH2, CH3 or phenyl, and i and j respectively represent 0, 1 or 2. In synthesis examples 13 and 14 of both cited applications, however, only products with the above formula in which i=1 and j=0, i.e. mono-phosphorylated melamine or guanamine, were described, and in fact by reacting 1 mol of DOPO-Cl, i.e. 10-chloro-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-one, or 1 mol of DPhPO-Cl, i.e. diphenylphosphoryl chloride, with 1 mol of melamine and heating to about 170° C. The analogous synthesis of compounds containing more than one phosphorus-containing DOPO or DPhPO group by reacting i+j mol of Q′Cl with 1 mol of melamine or guanamine was mentioned in general terms without, however, describing a specific method.
During the course of their research, the Applicants have discovered that this analogous reaction of up to 4 mol of DOPOCl or DPhPOCl with 1 mol of melamine or guanamine cannot produce the desired products if i and/or j is/are equal to 2, and in particular if the melamine is only derivatized at two of its three amino groups. Because of the reactivity of the hydrogen of an amino group which has already been mono-substituted with the respective phosphorus compound, i.e. a —NHQ′ group, then in theory it is not possible to di-substitute an amino group with Q′ without protecting the hydrogens of the third amino group, since —NH2 is much more reactive than —NHQ′ as regards phosphoryl chlorides.
Furthermore, the reaction times in Wang et al's method are very long. Thus, for example, once addition of the reagents has been completed, stirring times of 16 h (for DOPO-Cl) or 10 h (for DPhPO-Cl) are necessary in order to obtain essentially complete conversion, even though temperatures of around 170° C. are employed.
Thus, the aim of the invention is to provide an improved method for the production of such or similar compounds by means of which these compounds can be obtained in good yields after relatively short reaction periods and essentially without any disruptive side reactions.
2. Description of the Related Art
The invention accomplishes this aim by providing a method for producing compounds with the following formula (I):

wherein:                the residue R1 is selected from —NH2, —NH2-zAz as well as monovalent alkyl and aryl residues,        the residues A are each selected, independently of each other, from the following phosphoryl residues DOPO-, DPhPO- and DPhOPO-:        
and                the indices x, y and z each, independently of each other, represent 0 or 1, wherein at least one of the indices ≠0;        
in which, in a first step, melamine or, when R1 is an alkyl or aryl residue, the corresponding alkyl or aryl guanamine is reacted with one or more of the following phosphinyl chlorides DOP-Cl, DPhP-Cl and DPhOP-Cl:

in order to bind one or more phosphinyl residue(s) to the amino group(s) of the melamine or guanamine, after which in a second step, the bound phosphinyl residue(s) is(are) oxidized by reaction with an oxidizing agent to form the corresponding phosphoryl residue(s).
More precisely, in the first step, a compound with formula (II):

wherein the residue R2 is selected from —NH2 and monovalent alkyl and aryl residues, i.e. melamine or a guanamine, is reacted with one or more of the phosphinyl chlorides DOP-Cl, DPhP-Cl and DPhOP-Cl to form one or more compounds with the following formula (III):

wherein:                the residue R3 is selected from —NH2, —NH2-zBz and monovalent alkyl and aryl residues;        the residues B, independently of each other, are selected from the following phosphinyl residues DOP-, DPhP- and DPhOP-:        
and                the indices x, y and z are as hereinbefore defined;        
wherein in the second step, the compound(s) with formula (III) are oxidized by reaction with the oxidizing agent to form one or more compounds with formula (I).
By means of this novel method, not only can compounds with formula (I) be produced in good yields and essentially without side reactions, but also, melamines can be produced which are tri-substituted with the respective phosphorus-containing residue without problems, and specific mixtures of mono-substituted and di-substituted aminotriazines can also be obtained, as will be seen in the examples below. Furthermore, the reaction times compared with Wang et al's method could be substantially reduced, which is of distinct economic advantage having regard to the high reaction temperatures employed.
Without wishing to be bound by a specific theory, it is assumed that this due to the substantially greater reactivity of phosphinyl chlorides compared with the corresponding phosphoryl chlorides. However, the near-quantitative yields for production of the desired compounds was surprising. It is actually because of the high reactivity of the phosphinyl chlorides that more side reactions should have been expected to occur—especially at the preferred high reaction temperatures of up to 200° C. This means that it was unexpected that essentially, the respective desired product would be formed exclusively containing one phosphorus-containing residue per amino group; rather, a certain—albeit smaller—fraction with two phosphorus-containing residues on one amino group would have been expected to have been obtained. However, this latter case was not observed at all. Thus, the phosphinyl chlorides employed in the method of the invention have a sufficiently high reactivity to allow rapid bonding of exactly one phosphorus-containing residue to occur per amino group, but it is not sufficient to di-substitute the amino group, even when a molar excess of phosphinyl chloride is employed.
Furthermore, the phosphinyl chloride DOP-Cl used in the method of the invention for the production of DOPO derivatives is an industrial intermediate in the production of the conventional flame retardant DOPO, and thus is produced in large quantities, and hence is much cheaper and more readily available than the phosphoryl chloride DOPO-Cl which is used in Chun-Shan Wang's method.
The monovalent alkyl and aryl residues as options for the residues R1 to R3 are preferably —CH3 or —C6H5, i.e. methyl or phenyl, since these form conventional and readily obtainable guanamines—methyl guanamine and phenyl—and also benzo-guanamine. The method of the invention is, however, applicable to a broad range of substituents on the diaminotriazine nucleus, and hence the scope of protection should not be limited to these two preferred residues.
The phosphorus-containing intermediates obtained in the first step are largely stable to hydrolysis and thus can readily be isolated, for example by stirring into water. Furthermore, one-pot syntheses are possible for both steps. Both will be described in the exemplary embodiments below.
There are no particular limits as to the oxidizing agent, as long as it does not result in any unwanted side reactions of the reaction participants. In preferred embodiments, a peroxide is in fact used, as any excesses are easy to separate or destroy. In addition to hydrogen peroxide, H2O2, many other peroxides and hydroperoxides may be used. Even the oxidation of DOP to DOPO with ozone, which is known from the literature, is possible, but in the inventors' experiments, the results were not as good as with hydrogen peroxide or t-butyl hydroperoxide. Finally, H2O2 is preferred because of its greater stability and thus ease of handling.
Since HCl is given off in the reaction between phosphinyl chloride and the aminotriazine, the first step is preferably carried out in the presence of an acid scavenger in order to displace the reaction equilibrium towards the product side. The acid scavenger is preferably 1-methylimidazole, although clearly other compounds known to the skilled person, such as ammonia, alkyl- and aryl-amines and other nitrogen compounds such as trimethylamine, pyridine, imidazole and the like, or other bases such as alkali and alkaline-earth metal compounds, for example, may also be used. 1-methylimidazole has the great advantage that its hydrochloride melts at a temperature as low as 75° C. (while, for example, the non-methylated homologue imadazolium chloride has a melting point of 158-161° C.), and thus with a suitable choice for the solvent and the reaction temperature, a second liquid phase which can easily be separated is formed in addition to the reaction solution.
Both steps are preferably carried out in an organic solvent in order to ensure the homogeneity of the reaction and heat dissipation. There are no specific restrictions applying to the solvent, as long as it is chemically inert having regard to the reactions occurring therein and the starting products are soluble or at least dispersible therein. However, in the first step, for the reasons given above, it should have a boiling point which is substantially above 75° C., the DOP-Cl should have a sufficiently good solubility in it, and 1-methylimidazolium chloride should not dissolve in it. In accordance with the present invention, then, a relatively apolar, anhydrous solvent is preferably used, more preferably an aromatic hydrocarbon such as, for example, benzene, toluene, xylene or the like, in particular toluene. Preferably, the same solvent is used both for reaction of the aminotriazine with the phosphinyl chloride and also for the subsequent oxidation. However, different solvents may also be used, for example chloroform for the first step and toluene for the oxidation, etc.
In preferred embodiments of the invention, in the first step, the acid scavenger simultaneously acts as a solvent; hence in particular in this case, 1-methylimidazole acts both as an acid scavenger and as the solvent. In the second step, chloroform or toluene are preferred, in particular toluene because it is halogen-free.
Although the invention is not limited thereto, during the course of the inventors' research, it was shown that the first step should preferably be carried out at a temperature in the range 100° C. to 200° C., in order to ensure short reaction times, high conversions and good yields. In similar manner, for the second step, temperatures in the range 50° C. to 100° C. have proven advantageous.
The scope of the invention naturally also encompasses the direct products of the method of the invention, i.e. the compounds with formula (I) which are so produced.
Since the tri-substituted melamine, i.e. 2,4,6-tris(9,10-dihydro-9-oxa-10-oxo-10-phosphaphenanthrene-10-ylamino)-1,3,5-triazine (DOPO3-mel):

in which DOPO- is the residue
constitutes a novel chemical compound, in a further aspect, the invention pertains to protection of this product as well as to its use as a flame retardant. In the first flammability tests, the inventor shows this novel compound to have an exceptional effect as a flame retardant for plastic materials, in particular for polystyrene and epoxides.